1. Field of the Invention
This invention relates to curing agents suitable for use for curing epoxy resins to form storage stable epoxy resin compositions containing the curing agents. The present invention further relates to methods of curing epoxy resin compositions with the curing agents.
2. Description of Related Art
Epoxy resins are very widely used as matrices for fiber-reinforced composites. Such composites are used in industrial and aerospace applications as structural components, and for electronic circuit boards and in many other areas. Epoxy resins are also widely used as structural adhesives in the form of so-called film adhesives and as pastes and liquids.
There are a number of different reaction mechanisms which can be used to cure epoxy resins. These are well known in the art and are described in, for example, "The Chemistry and Technology of Epoxy Resins" by B. Ellis, Blackie Academic Press, Glasgow 1993, Chapter 2 "Curing Agents for Epoxy Resins." One known mechanism involves reacting the epoxy with a stoichiometric amount of an active hydrogen containing compound, for example a difunctional primary amine with each epoxy terminal reacting with a corresponding active hydrogen containing moiety. Another mechanism involves the utilization of an agent which behaves as an initiator for the ring opening polymerization of the epoxide ring. Thus the purpose of the initiator is to activate a terminal epoxide moiety on the epoxy resin by breaking open the epoxide ring and forming an unstable intermediate which can then react with a second epoxide terminal. Thus in this reaction mechanism one epoxide terminal effectively reacts with another epoxide terminal without there being needed a separate cross-linking molecule. The most common initiator is a tertiary amine of which imidazoles are particularly well known.
Thus imidazoles are known as very effective, highly active curing agents for epoxy resins. Many examples are commercially available, for instance, 2-methyl imidazole, 2-ethyl-4-methyl imidazole, 2-phenyl imidazole and 1-benzyl-2-methyl imidazole.
Generally, epoxy resins and corresponding curing agents are packaged in separate pots to avoid premature curing on storage. In use the cured composition is formed by mixing ingredients from the respective pots. However, some one pot epoxy compositions are known in the art. Examples of such compositions are compositions comprising the epoxy resin and imidazole curing agents. However, although these compositions are ideal for bringing about the rapid cure of epoxies at relatively low temperatures, the pot life of such one pot compositions is short, for example, on the order of one to three days at room temperature.
Conversely, other known one pot compositions have longer pot lives on the order of one or more weeks but these known compositions are disadvantageous in view of the high cure temperature that is required which is typically well over 100.degree. C.
The compounds 1-benzyl-2-methyl imidazole and 2-methyl imidazole are known curing agents for epoxides. However, when formulated in one pot compositions, these curing agents typically exhibit a pot life of only two to three days at room temperature.
Thus, although many imidazoles are ideal for bringing about the rapid cure of epoxy resins at relatively low temperatures, the combination of imidazole and epoxy resin must be used very quickly owing to the poor stability of the formulations.
In certain applications it is very important to have an epoxy resin formulation which will cure rapidly at relatively low temperature, for example, 60 to 80.degree. C., but which will nevertheless maintain an acceptably long out-life on storage at room temperature. Examples of such applications are in the form of very large pre-preg parts, for instance in the fabrication of large structures in the transportation industry. It is often essential in such cases to use the lowest practicable cure temperature to avoid the need for very large ovens. Arranging a simple awning over the laid-up structure and directing hot air at the surfaces to be cured may then effect cure. Because of the length of time taken to lay up the pre-preg sheets, a long out-life is needed, often several days to a week or more.
It is found that in practice that the existing imidazole curatives have sufficient latency to allow their use in such low temperature, long out-life applications. Whilst cure can often be achieved at the required low temperature, the latency is usually inadequate, often on the order of two to three days. An out-life of at least one week and preferably at least two weeks would be required in many cases. Conversely, imidazole curatives are available which possess latencies of one or more weeks, but then the minimum useful cure temperature will be much higher, and typically well over 100.degree. C.
In U.S. Pat. No. 3,756,984 there are described reaction products of glycidyl esters and imidazoles, particularly those in which there is a tertiary or quaternary carbon in the carboxylic acid from which the glycidyl ester is derived. No long chain alkyls are described as the chain is interrupted by an ester grouping. The emphasis is on increasing solubility of the products. There is no suggestion of the advantages of C.sub.12 -C.sub.18 chains.